Position a rack in the center of the oven and heat the oven to 350°F. Set the potatoes directly on the oven rack and bake until tender all the way through, 1 hour to 1 hour and 15 minutes. Transfer the potatoes to a work surface and let them cool for 10 to 15 minutes.
Examine each potato to see if there’s a way to halve it to give you two shallow, wide halves rather than taller, narrower ones. With a large chef's knife, slice each potato in half lengthwise, cutting cleanly— not sawing—so as not to tear the skin. Using a dishtowel or oven mitt to protect your hand from the heat, hold a potato half in one hand and gently scoop out the flesh with a spoon, leaving the shells 1/4 to 1/8 inch thick.Repeat with the remaining halves. Force all the flesh through a potato ricer or mash it with a potato masher; transfer it to a mixing bowl.
With a wooden spoon, stir 3 Tbs. of the butter, the crème fraîche, half-and-half, cheeses, bacon, salt, pepper to taste, and chives or scallions, if using, into the mashed potato flesh. Scoop the filling into the potato skins, compacting it lightly. For a rough-textured surface, mark it with the tines of a fork. Top each with bits of the remaining 1/2 Tbs. butter..
Heat the oven to 400°F. Arrange the potatoes on a baking sheet or in a large baking dish. Bake until heated through and beginning to brown in spots on top, 25 to 30 minutes (or 35 to 40 minutes if made ahead and refrigerated). Let sit for about 10 minutes before serving.
A main virtue of twice-baked potatoes is that you can make them ahead and reheat to serve—a real boon for entertaining. Once you've filled the potatoes, transfer them to a baking dish, cover tightly with plastic, and refrigerate for several hours or overnight. They can go directly from the refrigerator to the oven (obviously, remove the plastic wrap first). Once the potatoes are baked, they can sit for 10 to 15 min. before serving.